Apparatus for protecting a subscriber line interface circuit against overvoltages

ABSTRACT

An apparatus interfaces with a subscriber line interface circuit (1) and protects the interface circuit (1) against overvoltages exceeding a supply voltage source supplied to the subscriber line interface circuit. The apparatus includes first and second diodes (DA1, DB1) having respective anodes connected to first and second input wires (A, B). The cathodes of the first and second diodes are connected to a positive terminal of the supply voltage source. The apparatus further includes third and fourth diodes (DA2, DB2) having cathodes connected the first and second input wires (A, B). The anodes of the third and fourth diodes are connected to a cathode of a thyristor (TAB). The anode of the thyristor, in turn, is connected to the positive terminal of the supply voltage source, and the gate the thyristor is connected to a negative terminal of the supply voltage source.

TECHNICAL FIELD

The present invention relates to apparatus for protecting a subscriberline interface circuit against overvoltages appearing on its two inputwires and exceeding the voltage of the supply voltage source of thesubscriber line interface circuit.

BACKGROUND ART

It is known to protect subscriber line interface circuits againstovervoltages, primarily against overvoltages in the form oflightening-induced overvoltage pulses caused, with the aid of aso-called primary protector which may, for instance, consist in a sparkgap which functions to reduce the overvoltages on the subscriber line toa voltage of about one kilovolt. A so-called secondary overvoltageprotector is fitted to the subscriber line, to deal with these residualpulses of about one kilovolt. The secondary overvoltage protector mayhave the form of a known circuit marketed by General Semiconductor underthe trademark TRANSZORB, or a circuit marketed by RCA under thetrademark SURGECTOR.

Current standards require the secondary overvoltage protector to beearthed to the apparatus stand with a separate wire. However, this cancreate problems that are manifested in the form of residual pulses inthe order of magnitude of some hundred volts across a subscriber lineinterface circuit connected to the subscriber line. Modern lineinterface boards, in which the line interface circuit is a monolithicintegrated circuit, are very often broken-down by lightening, due toresidual pulses from the secondary overvoltage protector appearingacross the two input wires of the circuit board.

U.S. Pat. No. 4,456,940 teaches an overvoltage protector which includes,among other things, two diodes whose anodes are each connected to arespective input terminal of a subscriber line interface circuit andwhose cathodes are connected to earth, and further includes twothyristors whose cathodes are each connected to a respective inputterminal on the subscriber line interface circuit and whose anodes areconnected to earth. The thyristor gates are connected to the supplyvoltage of the subscriber line interface circuit. This protectivecircuit protects the subscriber line interface circuit againstovervoltages. However, the thyristors are relatively space-consuming anda reduction in their number would be a significant advantage. It issometimes necessary to match the thyristors, so that they will ignite orfire simultaneously, i.e. for the same voltage across the cathodes withequal voltage across the gate. However, the document does not describeseveral cascade-connected overvoltage protectors.

SUMMARY OF THE INVENTION

The object of the invention is to eliminate the effect on a subscriberline interface circuit of residual pulses that emanate from secondaryovervoltage protectors.

This object is achieved with apparatus according to the invention andhaving the characteristic features set forth in the following claims.

One advantage afforded by the present invention is that the number ofthyristors required in the tertiary overvoltage protector has beenreduced in comparison with the number required in the known protectors,therewith reducing the space required by the overvoltage protector.Another advantage is that matching between two or more thyristors toensure that the thyristors will be activated at one and the same voltagelevel is no longer necessary.

BRIEF DESCRIPTION OF THE DRAWING

The invention will now be described in more detail with reference to theaccompanying drawing, in which FIG. 1 is a block schematic whichillustrates a primary overvoltage protector circuit, a secondaryovervoltage protector circuit and a tertiary overvoltage protectorcircuit connected to a subscriber line interface circuit; and FIG. 2illustrates an embodiment of a tertiary overvoltage protector circuitaccording to the invention for a subscriber line interface circuit.

BEST MODE OF CARRYING OUT THE INVENTION

FIG. 1 is a block schematic illustrating an inventive total overvoltageprotector for a subscriber line interface circuit 1. The subscriber lineinterface circuit 1 has its input terminals LA and LB connected to apair of outlet terminals 5, 6 on a tertiary overvoltage protector 2. Thetertiary overvoltage protector 2 has two input terminals 7, 8 connectedin cascade to two output terminals 9, 10 of a secondary overvoltageprotector 3, which in turn has two input terminals 11, 12 connected incascade to two output terminals 13, 14 of a primary overvoltageprotector 4. The primary overvoltage protector 4 has two input terminals15, 16, each of which is connected to a respective resistor RA and RB,the resistances of which are normally in the order of 10 ohms and eachof which is connected to a respective input wire A and B of thesubscriber line interface circuit 1. The primary overvoltage protector 4is normally of a known kind, i.e. a spark gap, and the secondaryovervoltage protector 3 is normally of a known kind, such as the kinddesignated TRANSZORB or SURGECTOR. As shown in the Figure, the tertiaryovervoltage protector 2 and the subscriber line interface circuit 1 areconnected to system earth, whereas the secondary overvoltage protector 3is connected to apparatus earth.

Overvoltage pulses that are caused by lightening for instance, and whichoccur on the two input wires A and B of the circuit 1 are reduced by theprimary protector 4 to residual pulses in the order of one kilovolt,these residual pulses being reduced to the order of some hundred voltsby the secondary overvoltage protector. These residual pulses arrivingfrom the secondary protector 3 and having a voltage level which exceedsthe supply voltage to the subscriber line interface circuit areeliminated by the tertiary overvoltage protector 2.

The inventive tertiary overvoltage protector 2 is shown in broken linesin FIG. 2. The secondary and primary overvoltage protectors have beenomitted from the FIG. 2 illustration, and the tertiary overvoltageprotector 2 is shown connected directly to the resistors RA and RB. Theinput terminals LA, LB of the subscriber line interface circuit 1 areeach connected to the anode of a respective first diode DA1, DB1, thecathodes of which first diodes are connected to system earth. The inputterminals LA and LB are also each connected to the cathode of arespective second diode DA2 and DB2, wherein the anodes of the seconddiodes are connected to the cathode of a thyristor TAB which is commonto said second diodes and the anode of which is connected to systemearth and the gate of which is connected to a connection terminal on thevoltage-supply source of the interface circuit 1, said supply voltagesource having the potential -VB. The tertiary overvoltage protector 2thus includes the first diodes DA1 and DB1, the second diodes DA2 andDB2 and the thyristor TAB.

The subscriber line interface circuit 1 is supplied between earth andthe voltage -VB, which is normally -50V. The input terminals GND and VBBof the subscriber line interface circuit 1 are therefore connected tosystem earth and the potential -VB respectively.

Occurring overvoltages will be restricted in the following manner:

Positive overvoltages are restricted with the aid of the diodes DA1,DB1, whereas negative overvoltages are restricted to about 2V beneath-VB, in other words more negative than -VB, where 2V corresponds to thesum of the forward voltage drop of respective diodes DA2, DB2 and theforward voltage drop across the gate for turning on the thyristor TAB.The input terminals LA, LB are drawn to earth when the thyristor TABignites.

The inventive protective circuit is thus effective in eliminating theproblem associated with the breakdown of subscriber line interfacecircuits that are subjected to lightening induced overvoltage pulses.

I claim:
 1. An apparatus for protecting a subscriber line interface circuit against overvoltages exceeding a supply voltage source supplied to the subscriber line interface circuit, wherein said overvoltages occur on first and second input wires which are connected to said interface circuit via respective first and second input terminals of said interface circuit, wherein resistors are connected in series with respective said input wires, wherein said first and second input terminals are connected to anodes of respective first and second diodes, and wherein cathodes of the first and second diodes are connected to a positive terminal of the supply voltage source, further wherein said first and second input terminals are also connected to cathodes of respective third and fourth diodes, wherein anodes of said third and fourth diodes are connected to a cathode of a thyristor, wherein an anode of the thyristor is connected to the positive terminal of the supply voltage source and a gate of said thyristor is connected to a negative terminal of said supply voltage source.
 2. The apparatus of claim 1, wherein said negative terminal of the supply voltage source supplies a voltage values of approximately -50 volts.
 3. An apparatus for protecting a subscriber line interface circuit against overvoltages that exceed a supply voltage source supplied to said subscriber line interface circuit, wherein said overvoltages occur on first and second input wires which are connected to respective input terminals of a primary overvoltage protector, wherein resistors are connected in series with respective said input wires, wherein said primary overvoltage protector is connected in cascade to a secondary overvoltage protector and said secondary overvoltage protector is connected in cascade to a tertiary overvoltage protector, and said tertiary overvoltage protector is connected in cascade to the subscriber line interface circuit via first and second input terminals, wherein the secondary overvoltage protector is connected to apparatus earth, wherein said respective first and second input terminals of the subscriber line interface circuit are connected to anodes of respective first and second diodes and wherein cathodes of the first and second diodes are connected to a positive terminal of the supply voltage source, further wherein the tertiary overvoltage protector also includes a thyristor and third and a fourth diodes, wherein said respective input terminals of the subscriber line interface circuit are also connected to respective cathodes of said respective third and fourth diodes, wherein anodes of the third and fourth diodes are connected to a cathode of the thyristor, and wherein an anode of said thyristor is connected to the positive terminal of the supply voltage source and a gate of the thyristor is connected to a negative terminal of the supply voltage source.
 4. The apparatus of claim 3, wherein said negative terminal of the supply voltage source supplies a voltage values of approximately -50 volts. 